Since a motor for driving a printer that uses a platen roller as sheet conveying means generates heat, various heat radiation countermeasures have been proposed. In a small printer that utilizes a thermal head, in particular, a frame that is frequently used for heat radiation is small in size and heat capacity, and components are located so close to one another that the heat radiation environment is poor. In addition, heat is also radiated from a heat unit, and a thermal influence on a head must be taken into consideration. Thus, a heat radiation countermeasure for the motor is an essential factor.
Thermal printers with a heating structure are generally known, as described in Japanese Patent Application Laid-Open Nos. 7-0237324 and 2005-238658, for example.
In the thermal printer described in the above Japanese Patent Application Laid-Open No. 7-0237324, a driving force transmission mechanism for transmitting a decelerated rotation of a motor is configured by fixing a gear mounting member to the inside of an aluminum box member formed by bending an aluminum plate. A mounting flange of the motor is mounted on one outside surface of the aluminum box member so that heat generated from the motor can be radiated through the box member in close contact with the flange, thereby suppressing an increase in temperature of the motor.
However, the aluminum box member has so small a heat capacity that it is inevitably thermally saturated in a short time. Therefore, heat release from the motor balances with heat radiation from the box member at too high a temperature to ensure a satisfactory heat radiation effect.
In the thermal printer described in the above Japanese Patent Application Laid-Open No. 2005-238658, on the other hand, a drive unit that includes a motor mounted on a gear mounting member is attached to one side wall of a mainframe that supports a platen roller. Since the gear mounting member on which the motor is mounted is formed by die-cast working with zinc alloy, heat generated from the motor can be radiated through the gear mounting member.
Since the gear mounting member is a die-cast product of an alloy material, its heat radiation effect is higher than that of the aforesaid aluminum box member. Since the gear mounting member is a small member that is attached to the one side wall of the mainframe, however, a very high heat radiation effect cannot be expected of it. In addition, the gear mounting member is not positively designed to improve such radiation, in addition to forming it by die-cast working.